This invention relates to an antenna structure, and, more particularly, a novel conformal antenna structure having broadband characteristics as well as a radiation pattern and impedance characteristics that are essentially independent of frequency over a wide range.
In designing antenna structures, it should be kept in mind that the antenna designer must make the antenna perform a desired electrical function such as transmitting/receiving linearly polarized, right-hand circularly polarized, left-hand circularly polarized, etc., r.f. signals with appropriate gain, bandwidth, beamwidth, minor lobe level, radiation efficiency, aperture efficiency, receiving cross section, radiation resistance and other electrical characteristics. It is also necessary for these structures to be lightweight, simple in design, inexpensive and unobtrusive since an antenna is often required to be mounted upon or secured to a supporting structure or vehicle such as high velocity aircraft, missiles, and rockets which cannot tolerate excessive deviations from aerodynamic shapes. Of course, it is also sometimes desirable to hid the antenna structure so that its presence is not readily apparent for aesthetic and/or security purposes. Accordingly, the ideal electrical antenna should physically be very thin and not protrude on the external side of a mounting surface, such as an aircraft skin or the like, while yet still exhibiting all the requisite electrical characteristics.
Antennas that have very low profiles which may be flush mounted on a supporting surface are generally referred to as conformal antennas. As discussed, such an antenna must actually conform to the contour of its supporting surface, and, therefore, reduce or eliminate any turbulent effects that would result when such a device is mounted or secured to a vehicle and propelled through space. Conformal antennas may, of course, be constructed by several methods, but can be generally produced by rather simple photoetching techniques since such techniques offer ease of fabrication at a relatively low production cost.
Such conformal antennas or printed circuit board antennas, as they may be called, are formed by etching a single side of a unitary metallically clad dielectric sheet or electrodeposited film using conventional photoresist-etching techniques. Typically, the entire antenna structure may possibly be on 1/32 inch to 1/8 inch thick which minimizes cost and maximizes manufacturing/operating reliability and reproducibility. It can be appreciated that the cost of fabrication is substantially minimized since single antenna elements and/or arrays of such elements together with appropriate r.f. feedlines, phase shifting circuits and/or impedance matching networks may all be manufactured as integrally formed electrical circuits alone using low cost photoresist-etching processes commonly used to make electronic printed circuit boards. This is to be compared with many complicated and costly prior art techniques for achieving polarized radiation patterns as, for instance, a turnstile dipole array, the cavity backed turnstile slot array and other types of special antennas.
A resonant antenna is one which is an integral number of half-wavelengths. In a resonant antenna standing waves of current and voltage are established causing the maximum amount of radiated energy to be radiated as the antenna reactance for a particular frequency is lowest. Of course, an antenna need not exhibit resonant properties to operate satisfactorily. An antenna may operate and be designed to have approximate uniform current and voltage amplitudes along its length. Such an antenna is generally characterized as a traveling wave antenna and is nonresonant.
In general, an antenna is limited in the range of frequencies over which it effectively operates. An antenna may operate satisfactorily, of course, within a fixed frequency range with a signal that is narrower in its bandwidth and, generally, in the design of such an antenna there are no particular bandwidth problems. On the other hand, if a broadband antenna is required, there are often a number of difficulties that an antenna designer must overcome to produce a satisfactory operating antenna device. Under certain conditions, it is possible in a number of applications to actually use an essentially narrow-band antenna over a wide frequency range if allowance and provisions are actually made for modifying the antenna's dimensional characteristics or for adjusting the impedance matching transformer characteristics of the antenna. In many operations, however, it is necessary that an antenna structure having a fixed configuration operate over a very broad range of frequencies. Accordingly, a number of broadbanding techniques have been practiced to achieve this goal since an antenna having a broad bandwidth is highly desirable.
In considering bandwidth, there are generally two categories of parameters to be addressed: (1) the antenna radiation pattern, and (2) impedance characteristics. As regards the radiation pattern, parameters to be considered for designing a broadband antenna include the power gain, beamwidth, side-lobe level, beam direction and polarization and, as regards the impedance characteristics, parameters to be considered include input impedance, radiation resistance and antenna efficiency.
With respect to a resonant antenna, resistive loading of such an antenna provides a means to broaden its impedance bandwidth. In this regard, broadband dipole antennas have been made by making the thickness of the conducting element large relative to their length. Thus, broadbanding dipole structures have been simply accomplished by employing large diameter conductors rather than thinner ones. In this regard, biconical antennas belong to this general class and are generally considered to be broadband antennas. Nonetheless, resistive loading is not generally employed for antennas operating at high frequencies since conductor losses are usually exceeding small which, in turn, results in an antenna having an inadequate bandwidth.
Certain antennas having a wide variety of physical sizes and shapes are known to be frequency independent, often achieving bandwidths of at least 10 to 1 and substantially higher. In general, their broadband behavior includes both impedance and radiation pattern characteristics. Such frequency independent antennas, as they are called, generally exhibit a certain shape or pattern of geometric form. For such antennas there are certain structural patterns that are more or less repeated with changing dimensions. An illustrative example of this design characteristic is found in the so-called log-periodic dipole array antenna.
Although a number of such antennas are known and include the Beverage antenna or wave antenna, the rhombic antenna and the aforementioned log-periodic antenna, all these devices are relatively large and require substantial space.
U.S. Pat. No. 2,942,263 to Baldwin teaches a conventional notch antenna device. Further, U.S. Pat. No. 2,944,258 to Yearout, et al., discloses a dual-ridge antenna having a broad bandwidth. U.S. Pat. No. 2,985,879 to Du Hamel discloses a frequency independent antenna. The Du Hamel antenna is formed of a conducting material having an outline of a pair of intersecting lines serve at the feed point. The edges of the antenna are provided with a plurality of alternating slots and teeth that are dimensioned proportionally to their distance from the feed point. U.S. Pat. No. 3,836,976 to Monser, et al., disclosed a broadband phase array antenna formed by pairs of mutually orthogonal printed radiating elements, each one of such elements having a flared notch formed therein. Further, U.S. Pat. No. 4,500,887 to Nester discloses a broadband radiating element designed to provide a smooth, continuous transition from a microstrip feed configuration to a flared notch antenna.
A conventional notch antenna device 10 is shown in FIG. 1 and consists of a metallization 11 situated on and integrally connected to a dielectric substrate 13. The notch antenna device 10 has a mount 14 and a narrow slot 15 that are interconnected by a gradual transition as shown in FIG. 1. It is to be noted that a cavity 16 is formed at the base of the slot line 15, the cavity 16 being required for impedance matching the antenna to a transmission line. The cavity 16 places, nonetheless, a limitation on the ratio of high to low frequencies that the notched antenna device 10 can properly receive or transmit. The radiation pattern is unidirectional and generally provides bandwidth usually not exceeding about 4:1.